This invention relates to a process for deashing coal by selectively flocculating finely divided coal particles in an aqueous suspension thereof mixed with particles of inorganic impurities.
Petroleum has long been consumed as a major energy source because of its low price, high heat value and easy handling in transportation and combustion. Its increasing price and shortage of resources in recent years, however, have led attempts to utilize coal again as a substitute energy source for petroleum.
As is well-known, natural coal generally contains in addition to the carbonaceous content from 5 to 25% of ash contents composed of a major proportion of clay ashes such as silica and alumina, and a minor proportion of various metal oxides and sulfides. These ash contents leave a large quantity of unburned residue and produce environmentally harmful substances when combusted. It is for this reason that a high ash content greatly decreases the value of coal as a fuel.
In order to deash natural coal as much as possible and improve its value, several methods have been known including the heavy media separation process, the floatation process, the oil agglomeration process and the magnetic separation process. Among them, the most effective process is the oil agglomeration process in which an amount of a binding oil is added to an aqueous slurry of finely divided coal particles mixed with impurity particles to selectively agglomerate coal particles into pellets. This process requires a significant quantity of oil and energy for pelletizing the coal particles and the deashing rate achievable by this process does not exceed 50-60%. Another disadvantage of this process is the fact that the resulting deashed coal particles are in the form of a mixture with oil which is less convenient than aqueous slurries in transporting and combusting as such. Various attempts have been made, therefore, to obviate these and other defects by, for example, adding an emulsifying agent, an inorganic electrolyte or an oil-soluble polymer in combination with the oil binder, adding the oil binder as an emulsion or stepwise. Experiments have shown that the results of these attempts are far less than would be satisfactory.
Japanese laid-open patent application No. 54-16511 discloses a direct deashing process wherein ash particles are selectively sedimentated by adding to an aqueous slurry of finely divided coal particles a dispersing agent such as a water-soluble polyacrylate or polyphosphate. This process utilizes the difference between sedimentating speeds of the ash particles and the coal particles under the gravity but is difficult to operate satisfactorily in practice.
Japanese laid-open patent application No. 56-111062 discloses a deashing process of coal by chemically graft-polymerizing an unsaturated monomer with coal particles to render the coal particles more lipophilic and recovering the same. This process requires additional reagents and also cumbersome operations making its commercial application unsuitable.
It has been proposed to selectively flocculate a variety of mineral particles including coal using a water-soluble polymer having hydrophobic groups such as high molecular weight (500,000) polyethylene glycol and acrylamide-methyl acrylate copolymer. However, the requisite properties and conditions required for the flucculant used in this technique, particularly for use in coal have not been fully revealed.
It is, therefore, the major object of the present invention to provide a process for deashing coal by the selective flocculation technique which can avoid the foregoing defects inherent in the prior art processes and achieve a high deashing rate with a simple operation at a low reagent consumption.